1. Field of the Invention
This invention relates to blow molded plastic containers, and in particular to containers that require enhanced top load strength characteristics, especially in the region about the finish portion.
2. Description of the Related Technology
Many products that were previously packaged using glass containers are now being supplied in plastic containers. Polyethylene terephthalate (PET) thermoplastic resins are polyester materials that provide clarity and transparency that are comparable to glass. PET possesses the processing characteristics, chemical and solvent resistance and high strength and impact resistance that are required for packaging products such as juices, soft drinks and water. PET containers are lightweight, inexpensive, and recyclable and can be economically manufactured in large quantities. They will not shatter and create potentially dangerous shards when dropped, as a glass container may.
PET containers have conventionally been manufactured using the stretch blow molding process. This involves the use of a pre-molded PET preform having a threaded portion and a closed distal end. The preform is first heated and then is longitudinally stretched and subsequently inflated within a mold cavity so that it assumes the desired final shape of the container. As the preform is inflated, it elongates and stretches, taking on the shape of the mold cavity. The polymer solidifies upon contacting the cooler surface of the mold, and the finished hollow container is subsequently ejected from the mold.
Another well-known process for fabricating plastic containers is the extrusion blow molding process, in which a continuously extruded hot plastic tube or parison is captured within a mold and inflated against the inner surfaces of the mold to form a container blank. In such systems, the mold is typically designed to travel at the speed at which the extruded parison is moving when it closes on the parison so that the process can operate on a continuous basis. There are several different types of extrusion blow molding machines, including shuttle molds that are designed to travel in a linear motion and extrusion blow molding wheels that travel in a rotary or circular motion.
Extrusion blow molding is typically used to form plastic containers, such as motor oil containers, from nontransparent materials such as polyolefin or polyethylene. In the past, it was not typical to use extrusion blow molding to fabricate PET containers, because no commercially available PET material provided the required melt strength for extrusion blowmolding in addition to being compatible with standard PET recycling processes. More recently, however, extrudable PET (EPET) materials have been made commercially available that can be processed at temperatures and conditions similar to standard PET and that provide the required melt strength for extrusion blow molding. Such materials have higher melt temperatures than the polyethylene or polyolefin materials that are typically used with extrusion blowmolding. A limited number of PET containers that are fabricated using extrusion blow molding have now been commercially introduced.
Despite the advantages of plastic materials such as PET, glass containers are still prevalent for certain products, particularly those that require a substantial amount of column or top load strength so that the structural integrity of the container is not compromised when the containers are stacked in boxes or pallets and subjected to substantial vertical compressive forces. In many plastic container designs, the neck and shoulder portions are the weakest link in terms of the column strength of the container. The column strength of the container may also be reduced in container designs that have a main body portion with a broad upper surface adjacent to the finish portion of the container that does not slope sharply downward. In such container designs, downward force applied to the finish portion may cause the upper surface of the container to deflect downwardly and possibly buckle.
In order to minimize material costs, it is desirable to make the sidewall of these container portions, as with the rest of the container, as thin as possible. However, such lightweighting comes at the expense of container strength, and in particular column strength.
In the packaging of certain types of beverages, a rinsing agent is applied to the inner surfaces of the container, which must be drained from the container prior to filling the container with product. Any structural reinforcement that is provided to the upper portions of the container ideally should not interfere with efficient drainage of the rinsing agent from the container.
A need exists for a PET container that can be manufactured using an extrusion blow molding process that exhibits superior column strength, particularly in the upper regions of the container that are adjacent to the finish portion, and that does not adversely impact drainage from the container.